Free-flowing, hydrophobic starches are known in the art and have been used as dusting materials, dry lubricants, and detackifying agents. U.S. Pat. No. 2,961,339 (Wolff et al.) discloses a free-flowing modified starch and methods for preparing the starch by treating with alkyl siliconates. Wolff '339 purportedly discloses a siliconated granular starch which is water-repellant and completely free of undesirable alkalinity. Wolff '339 states that an improved starch is made by drying the starch before washing, so the water-repellant or hydrophobic quality is fixed but the alkalinity is not fixed and may be removed by washing.
U.S. Pat. No. 3,071,492 (Satterly) discloses a free-flowing hydrophobic starch and methods of manufacturing the starch by treating starch in a water slurry with a water-soluble silicone such as monosodium methyl siliconate. Satterly '492 states that its resultant product has dry flow characteristics not found in any raw starch or other starch product, and the flow characteristics resemble those of a fluid rather than a solid.
Wolf '339 and Satterly '492 do not expressly discuss the alkalinity of the slurry or resulting product. In the methods described by both Wolff '339 and Satterly '492, the aqueous mixture of the starch and sodium methyl siliconate would have a highly alkaline pH (e.g., a pH of 12 or higher) due to the siliconate. The free-flowing, hydrophobic starches would have a highly alkaline dry product pH.
Canadian Patent No. 726,667 (Speakman) describes free-flowing, hydrophobic starch and methods of preparation. Speakman '667 discusses a method of treating granule starch with an alkali metal alkyl siliconate to produce a powdered product having high mobility and hydrophobicity and possessing free acidity. Speakman '667 states that a highly mobile, hydrophobic starch product can be produced by mixing an aqueous solution of an alkali metal alkyl siliconate with an aqueous suspension of granular starch, adjusting the pH of the mixture to a low value by the addition of an acid, filtering off the starch without washing, drying the starch cake, and grinding to reduce the product to a powder.
Speakman '667 teaches that Wolff '339 and Satterly '492 operated at a highly alkaline pH and produced highly alkaline starch products. In discussing Wolff '339, the Speakman '667 patent states, “Even after washing and redrying, the products exhibit a high alkalinity.” In discussing Satterly '492, the Speakman '667 patent states, “Finally, the free-flowing hydrophobic products made by this method have appreciable alkalinity, as shown by pH values of 10 or greater of mixtures of the product and water after vigorous shaking.”
The Speakman '667 patent discloses adjusting the pH of an aqueous mixture of siliconate and starch to a low value by the addition of an acid. In a specific example, the pH of the mixture was adjusted to 2.5 using hydrochloric acid. The resulting starch had a dry product pH of 2.6. The Speakman '667 patent states, “It is possible to provide free-flowing hydrophobic starches having acidic or alkaline levels anywhere in a wide range by intimately mixing finely powdered alkaline products made by the methods of the prior art and acidic products made by the present invention.”
The known methods of preparing free-flowing hydrophobic starch products result in starch products which are highly alkaline or highly acidic when dispersed in water. The high alkalinity or high acidity of known starch products when dispersed in water poses the possibility that these starch products might, under some conditions, corrode some materials with which they come into contact. The potential for corrosion limits the desirability of these known methods and compositions where corrosivity is a concern.
While the methods listed above are useful for preparing free-flowing hydrophobic starch, there is a need for a free-flowing hydrophobic starch whose dry product pH is near-neutral.